1. Field of the Invention
The present invention generally relates to a light emitting device, and more particularly, to a light emitting device having a light source composed of light emitting diodes (LEDs).
2. Description of Related Art
Nowadays, light emitting diodes (LEDs) are increasingly adopted in various products as the light sources thanks to the small volume, low power consumption, long lifetime, and low cost of the LEDs. In addition, LED requires a very low operating voltage (only 1.5-3V), spontaneously emits light, and offers certain luminous intensity that is adjustable through the voltage or current. Besides, LED offers high shock resistance and long lifetime (100,000 hours). Thus, LED is broadly used in various terminal equipments, such as vehicle headlamps, traffic signal lamps, text displays, signboards, large-screen video displays, normal lightings, building lightings, and LCD backlights.
In an application wherein a plurality of LEDs is used as a light source, the LEDs are connected with each other in series so that the same current flowing through each LED and accordingly each LED has the same luminous intensity. However, when one of the LEDs is open-circuited, all the LEDs stop emitting light since the current cannot pass through. On the other hand, the LEDs may also be connected with each other in parallel. However, in this case, when one of the LEDs is short-circuited, almost all the current passes through the short-circuited LED. As a result, the other LEDs cannot emit light anymore due to the extremely low driving current.
FIG. 1 is a circuit diagram of a conventional light emitting device. Referring to FIG. 1, the light emitting device 100 includes a voltage source 110 and a plurality of LEDs 120, wherein these LEDs 120 present a series-parallel connection. The voltage source 110 supplies an operation voltage Vcc for driving the LEDs 120 to emit light. When one of the LEDs 120 is open-circuited, the current can still pass through other LEDs 120 connected in parallel so that the other LEDs 120 can still emit light. When one of the LEDs 120 is short-circuited, those LEDs 120 connected in parallel are affected while those LEDs 120 connected in series can still emit light. Similar structures of the light emitting device 100 are disclosed in U.S. Pat. Nos. 5,490,049 and 6,095,661, and the details of the light emitting device 100 can be referred to these patents.
However, even though the light emitting device 100 can still emit light when one of the LEDs 120 fails (short-circuited or open-circuited), the electrical character (for example, the current or voltage) of the entire light emitting device 100 changes due to the change of circuit structure caused by the failure. In other words, the luminous intensity of the entire light emitting device 100 changes when one of the LEDs 120 fails. In particular, when one of the LEDs 120 is short-circuited, all those LEDs 120 connected in parallel are affected (i.e., the entire row of LEDs 120 stop emitting light). Thus, a short-circuited LED 120 brings a greater impact than an open-circuited LED 120.